Burner control system with primary safety switch

Abstract

A primary side of a burner control circuit includes a triac switch for applying a line voltage to a burner motor and an igniter in response to a signal from a first optical coupler, and a circuit breaker responsive to a second optical coupler for disconnecting the primary from the line voltage. A low voltage or secondary side of the control circuit, which is responsive to a thermostat, contains a light sensitive element for detecting a flame at the burner and the light sources for both optical couplers and, as such, is effective to control both the triac and the circuit breaker. The secondary also contains an electrical timing circuit for causing the triac to shut the motor and igniter off when, a predetermined time after a call for heat by the thermostat has elapsed, a flame has not been detected in the furnace. In addition, the burner control circuit includes a fail-safe capability for opening the circuit breaker if the triac has failed into a permanently closed condition and there is no call for heat or the burner flame has gone out; thereby preventing the system from overheating or filling with fuel.

I claim: 
     
       1. A fail-safe operative electrical power circuit for energizing a load comprising: a power switch means for alternately connecting and disconnecting an electrical power source to a load in response to a turn-on signal and a turn-off signal applied to said power switch means;   a circuit breaking means operable to electrically disconnect said power source from said power switch means and said load in response to a trip current;   an electronic switch means in electrical series relationship with said power switch means operable to apply said trip current to said circuit breaking means, said electronic switch means having a control input for switching said electronic switch means between a conducting condition and a nonconducting condition, said trip current being applied to said circuit breaking means when said electronic switch means is in said conducting condition and said off signal is being applied to said power switch means, said power switch means being in a conducting condition; and   input means electrically connected to said control input for switching said electronic switch means to a nonconducting condition when said turn-on signal is applied to said power switch means.   
     
     
       2. A fail-safe operative circuit according to claim 1 wherein said input means is a photoresistive element having a resistance value varying between a minimum resistance value and a maximum resistance value. 
     
     
       3. A fail-safe operative circuit according to claim 2 wherein said photoresistive element is approximately at said maximum resistance value when said electronic switch means is in said conducting condition. 
     
     
       4. A fail-safe operative circuit according to claim 3 wherein said circuit breaking means includes an actuating element in electrical series relationship with said electronic switch means and said power switch means and a normally closed switch connected between said power source and said load, said actuating element causing said normally closed switch to move to an open non-conductive condition in response to said trip current applied to said actuating element. 
     
     
       5. A fail-safe operative circuit according to claim 4 wherein said electronic switch means is a gate-controlled semiconductor device having power terminals including an anode and a cathode and a control input including a gate, one of said power terminals being directly connected to said actuating element, said gate being connected to said photoresistive element. 
     
     
       6. A fail-safe operative electrical power circuit for energizing a load including a fuel burner motor and a fuel igniter, said circuit comprising: a triac switch alternating between a conducting condition and a nonconducting condition, said triac switch having a triac gate responsive to a turn-on signal causing said triac switch to switch to said conducting condition and to a turn-off signal causing said triac switch to switch to said non-conducting condition said signals being applied to said triac gate;   a thermal circuit breaker having a normally closed switch and a heat sensitive element operable to open said normally closed switch when a trip current flows through said heat sensitive element, said normally closed switch being series connected between an electrical power source and said triac switch;   a gate-controlled semiconductor device in electrical series relationship with said triac switch, said device having a gate for switching said device between a conducting condition and a non-conducting condition, said trip current being provided by said gate-controlled semiconductor device in said conducting condition when said off signal is being applied to said triac gate said triac switch being in said conducting condition; and   a photoresistive element connected to said gate, said photoresistive element having a resistance value varying between a minimum resistance value and a maximum resistance value, said photoresistive element being approximately at said maximum resistance value when said gate-controlled semiconductor device is in said conducting condition, said photoresistive element being shunted by a resistor.